It frequently happens that when two men have been engaged in any scene of action, the one shall remember it much better than the other, and shall have all the difficulty in the world to make his companion recollect it. He runs over several circumstances in vain; mentions the time, the place, the company, what was said, what was done on all sides; till at last he hits on some lucky circumstance, that revives the whole, and gives his friend a perfect memory of everything. Here the person that forgets receives at first all the ideas from the discourse of the other, with the same circumstances of time and place; though' he considers them as mere fictions of the imagination. But as soon as the circumstance is mentioned, that touches the memory, the very same ideas now appear in a new light, and have, in a manner, a different feeling from what they had before. Without any other alteration, beside that of the feeling, they become immediately ideas of the memory, and are assented to.
Since, therefore, the imagination can represent all the same objects that the memory can offer to us, and since those faculties are only distinguished by the different feeling of the ideas they present, it may be proper to consider what is the nature of that feeling. And here everyone will readily agree, that the ideas of the memory are more strong and lively than those of the fancy.
A painter, who intended to represent a passion or emotion of any kind, would Endeavour to get a sight of a person actuated by a like emotion, in order to enliven his ideas, and give them a force and vivacity superior to what is found in those, which are mere fictions of the imagination. The more recent this memory is, the clearer is the idea; and when after a long interval he would return to the contemplation of his object, he always finds its idea to be much decayed, if not wholly obliterated. We are frequently in doubt concerning the ideas of the memory, as they become very weak and feeble; and are at a loss to determine whether any image proceeds from the fancy or the memory, when it is not drawn in such lively colors as distinguish that latter faculty. I think, I remember such an event, say one; but am not sure. A long tract of time has almost worn it out of my memory, and leaves me uncertain whether or not it is the pure offspring of my fancy.
And as an idea of the memory, by losing its force and vivacity, may degenerate to such a degree, as to be taken for an idea of the imagination; so on the other hand an idea of the imagination may acquire such a force and vivacity, as to pass for an idea of the memory,, and counterfeit its effects on the belief and judgment. This is noted in the case of liars; who by the frequent repetition of their lies, come at last to believe and remember them, as realities; custom and habit having in this case, as in many others, the same influence on the mind as nature, and infixing the idea with equal force and vigor.
Thus it appears, that the belief or assent, which always attends the memory and senses, is nothing but the vivacity of those perceptions they present; and that this alone distinguishes them from the imagination. To believe is in this case to feel an immediate impression of the senses, or a repetition of that impression in the memory. It is merely the force and liveliness of the perception, which constitutes the first act of the judgment, and lays the foundation of that reasoning, which we build upon it, when we trace the relation of cause and effect.
(From A TREATISE OF HUMAN NATURE, BOOK I.Of the Understanding, PART III. OF KNOWLEDGE AND PROBABILITY by David Hume)